Fuel cells are generally classified, according to operating temperature and electrolyte, into an alkaline fuel cell (AFC), a phosphoric acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid oxide fuel cell (SOFC), a polymer electrolyte membrane fuel cell (PEMFC) and a direct methanol fuel cell (DMFC). Among them, the polymer electrolyte membrane fuel cell and the direct methanol fuel cell, which have excellent mobility, are receiving a great deal of attention as power sources.
Generally, a solid polymer fuel cell comprises a gas diffusion electrode layer disposed on both sides of a polymer electrolyte membrane, an anode as a negative electrode (reduction), and a cathode as a positive electrode (oxidation). In this fuel cell, water is produced by a chemical reaction in the polymer electrolyte membrane, and energy produced by this reaction is converted into electrical energy.
However, in the solid polymer electrolyte fuel cell, hydrogen peroxide (H2O2) is produced as a result of side reactions during the reduction of oxygen in the cathode. Hydrogen peroxide or peroxide radicals, produced in the cathode electrode layer, can deteriorate the electrolyte of the cathode electrode layer or the polymer electrolyte membrane adjacent thereto. In addition, if a phenomenon (crossover) occurs in which oxygen molecules pass from the cathode through the polymer electrolyte membrane to the opposite electrode, hydrogen peroxide or peroxide radicals will also be produced in the anode, potentially resulting in deterioration of the electrolyte of the anode electrode layer.
Ion conductive polymer electrolyte membranes developed in view of such problems are mostly perfluorinated polymer electrolyte membranes and are commercially available as Nafion (DuPont, USA), Aciplex-S membrane (Asahi Chemicals), Dow membrane (Dow Chemicals), Flemion membrane (Asahi Glass), etc.
Commercially available perfluorinated polymer electrolyte membranes have chemical resistance, oxidation resistance and excellent ion conductivity, but are costly and pose environmental problems due to the toxicity of intermediate products generated during the production thereof.
In order to overcome such drawbacks of the perfluorinated polymer electrolyte membranes, polymer electrolyte membranes comprising a carboxyl group, a sulfonic acid group or the like introduced into an aromatic ring polymer have been studied. Examples of such perfluorinated polymer electrolyte membranes include sulfonated polyarylether sulfone (Journal of Membrane Science, 1993, 83, 211), sulfonated polyetherether ketone (Japanese Patent Laid-Open Publication No. Hei 6-93114, and U.S. Pat. No. 5,438,082), and sulfonated polyimide (U.S. Pat. No. 6,245,881).
Accordingly, the present invention discloses a sulfonated aromatic monomer for preparing the above-described polymer introduced with a sulfonic acid group.
Generally, the sulfonated aromatic monomer is prepared according to the following reaction scheme 1. Specifically, as shown in reaction scheme 1, a target compound 1 is prepared by allowing an aromatic monomer 3 to react with fuming sulfuric acid to obtain a sulfonated compound 2 and adding a salt (NaCl) thereto.

However, as shown in FIG. 1 showing a conventional process for purifying a sulfonated aromatic monomer, many process steps are used to obtain the target compound with high purity, and thus the conventional process is disadvantageous in terms of yield and cost.
More specifically, the sulfonated aromatic monomer is to provided by a known method (Journal of polymer science: Part A: polymer chemistry, 2003, 41, 2264-2276). In the known method, a salt precipitation step is carried out twice, resulting in a decrease in yield. Also, because a strong base (NaOH) is used in a neutralization process, a change in the pH of the reaction product occurs even when the amount of the base slightly changes, and thus it is not easy to control the pH. In addition, the use of the strong base (NaOH) causes side reactions. Moreover, a recrystallization process is carried out in a 50:50 (v/v) mixture of alcohol and water. Because the desired compound dissolves in water, but does not easily dissolve in alcohol, it does not easily dissolve in the water/alcohol mixture, and thus the mixture can be used as a recrystallization solvent. However, for recrystallization, the water/alcohol mixture should be used in an amount corresponding to about 30-40 times that of the target compound 1. Thus, the loss of the solvent will result, and the salt (NaCl) can be recrystallized due to the use of excessive amount of the solvent.
Accordingly, the present inventors have made extensive efforts to overcome the problems of the conventional process for preparing the sulfonated aromatic monomer, have found a method for synthesizing a highly pure sulfonated aromatic monomer, which comprise a reduced number of salt precipitation steps, a simplified recrystallization process, and a purification process which is carried out using an easily available and stable chemical substance under mild conditions, thereby completing the present invention.